在 Android 6.0 或者之后的版本中要主动请求用户授权，具体方法，可以参考我之前写的这篇文章
《Android 6.0+ SMSManager 使用示例 发送短信》

实例的代码如下：

import android.app.Activity;
import android.content.ContentUris;
import android.content.Intent;
import android.database.Cursor;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.net.Uri;
import android.os.Build;
import android.os.Bundle;
import android.os.Environment;
import android.provider.DocumentsContract;
import android.provider.MediaStore;
import android.util.Log;
import android.view.KeyEvent;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Objects;

public class MainActivity extends Activity {

    private static final String TAG = "MainActivity";

    public static final int TAKE_PHOTO = 1;
    public static final int CROP_PHOTO = 2;
    public static final int CHOOSE_PHOTO = 3;

    private Button takePhoto;
    private Button chooseFromAlbum;
    private ImageView picture;
    private Uri imageUri;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        takePhoto = findViewById(R.id.takePhoto);
        chooseFromAlbum = findViewById(R.id.chooseFromAlbum);
        picture = findViewById(R.id.picture);
        takePhoto.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                File outputImage = new File(Environment.getExternalStorageDirectory(),
                        "output_image.jpg");
                try {
                    if (outputImage.exists() && outputImage.delete()) {
                        Log.d(TAG, "old image is deleted.");
                    }
                    if (outputImage.createNewFile()) {
                        Log.d(TAG, "new image file created.");
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }

                imageUri = Uri.fromFile(outputImage);
                Intent intent = new Intent("android.media.action.IMAGE_CAPTURE");
                intent.putExtra(MediaStore.EXTRA_OUTPUT, imageUri);
                startActivityForResult(intent, TAKE_PHOTO);
            }
        });
        chooseFromAlbum.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent("android.intent.action.GET_CONTENT");
                intent.setType("image/*");
                startActivityForResult(intent, CHOOSE_PHOTO);
            }
        });
    }

    @Override
    public boolean onKeyUp(int keyCode, KeyEvent event) {
        if (keyCode == KeyEvent.KEYCODE_BACK) {
            moveTaskToBack(true);
        }
        return super.onKeyUp(keyCode, event);
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        switch (requestCode) {
            case TAKE_PHOTO:
                if (resultCode == RESULT_OK) {
                    Intent intent = new Intent("com.android.camera.action.CROP");
                    intent.setDataAndType(imageUri, "image/*");
                    intent.putExtra("scale", true);
                    intent.putExtra(MediaStore.EXTRA_OUTPUT, imageUri);
                    startActivityForResult(intent, CROP_PHOTO);
                }
                break;
            case CROP_PHOTO:
                if (resultCode == RESULT_OK) {
                    try {
                        Bitmap bitmap = BitmapFactory.decodeStream(
                                getContentResolver().openInputStream(imageUri));
                        picture.setImageBitmap(bitmap);
                    } catch (FileNotFoundException e) {
                        e.printStackTrace();
                    }
                }
                break;
            case CHOOSE_PHOTO:
                if (resultCode == RESULT_OK) {
                    if (Build.VERSION.SDK_INT >= 19) {
                        handleImageOnKitKat(data);
                    } else {
                        handleImageBeforeKitKat(data);
                    }
                }
                break;
            default:
                break;
        }

        super.onActivityResult(requestCode, resultCode, data);
    }

    private void handleImageOnKitKat(Intent data) {
        String imagePath = null;
        Uri uri = data.getData();
        if (null != uri && DocumentsContract.isDocumentUri(this, uri)) {
            String docId = DocumentsContract.getDocumentId(uri);
            if (Objects.equals("com.android.providers.media.documents", uri.getAuthority())) {
                String id = docId.split(":")[1];
                String selection = MediaStore.Images.Media._ID + "=" + id;
                imagePath = getImagePath(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, selection);
            } else if (Objects.equals("com.android.providers.downloads.documents", uri.getAuthority())) {
                Uri contentUri = ContentUris.withAppendedId(
                        Uri.parse("content://downloads/public_downloads"), Long.valueOf(docId));
                imagePath = getImagePath(contentUri, null);
            }
        } else if ("content".equalsIgnoreCase(uri.getScheme())) {
            imagePath = getImagePath(uri,null);
        }
        displayImage(imagePath);
    }

    private void handleImageBeforeKitKat(Intent data) {
        Uri uri = data.getData();
        String imagePath = getImagePath(uri, null);
        displayImage(imagePath);
    }

    private String getImagePath(Uri uri, String selection) {
        String path = null;
        Cursor cursor = getContentResolver().query(uri, null, selection,
                null, null);
        if (null != cursor) {
            if (cursor.moveToFirst()) {
                path = cursor.getString(cursor.getColumnIndex(MediaStore.Images.Media.DATA));
            }
            cursor.close();
        }
        return path;
    }

    private void displayImage(String imagePath) {
        if (null != imagePath) {
            Bitmap bitmap = BitmapFactory.decodeFile(imagePath);
            picture.setImageBitmap(bitmap);
        } else {
            Toast.makeText(this, "Failed to get image.", Toast.LENGTH_SHORT).show();
        }
    }
}

本文参考自 《Android 第一行代码》